1. Field of the Invention
The present invention is directed generally to navigation systems and, particularly, to an improved system and method for position, velocity, orientation or angular rate sensing.
2. Description of the Related Art
Global navigation satellite system (GNSS) sensors are used in vehicles such as aircraft to determine vehicle position, velocity, orientation (attitude), and angular rate.
Use of GNSS position sensors to determine vehicle position is well known. GNSS position sensors can also be used to determine a velocity estimate by solving a set of range-rate equations or by smoothing (curve fitting) a set of position measurements.
A GNSS position sensor typically includes an antenna and an RF coaxial cable coupling the antenna to a GNSS signal processing unit. The GNSS antenna generally includes an antenna element and associated filtering and amplification electronics. Position is sensed at the phase center of the antenna (typically close to the geometric center of the antenna element). Typically, GNSS sensing provides bandwidth up to about 10 Hz.
GNSS orientation (attitude) sensors are also known (Orientation of a rigid body in space is defined by three (3) independent parameters. While various specifications of these parameters are possible, heading, pitch and roll are commonly used.).
An exemplary GNSS attitude sensing system 100 is shown in FIG. 1A. Typically, a GNSS attitude sensing system includes a plurality of GNSS antennas 102a-102d coupled via coaxial cables 104a-104d to a GNSS receiver unit 106. The relative positions of the antennas are used to derive a vehicle orientation. In addition, a GNSS attitude sensing system can generate angular rate measurements by solving a set of range-rate equations or by smoothing (curve-fitting) a set of attitude measurements.
Generally, the antennas are attached to the receiver via coaxial cables. To eliminate signal-to-noise (SNR) losses in the coaxial cable, low-noise amplification (LNA) and filtering electronics are placed in the GNSS antennas 102a-102d. The coaxial cable is used to transmit power from the receiver electronics to the antenna LNA electronics and to transmit the bandlimited GNSS signal to the receiver unit 106, where further amplification, filtering and signal processing is performed.
Inertial sensors, such as accelerometers and angular-rate sensors, may be used either alone or in conjunction with GNSS sensors to determine changes in position, velocity, orientation, and angular-rate. Change in position, for example, can be determined by twice integrating a set of accelerometer measurements; change in velocity can be determined by once integrating a set of accelerometer measurements. Similarly, an angular-rate sensor can directly measure angular-rate. The change in orientation can then be derived from integrating the angular rate measurement.
Inertial sensors are typically deployed in an inertial measurement unit (IMU) that houses, for example, an accelerometer, angular-rate, temperature and related sensors, as well as associated power supply, sampling filtering, and computational electronics. The IMU is typically located close to the center of gravity of the vehicle; the mounting orientation within the vehicle is an important installation constraint.
Returning to FIG. 1A, the system 100 includes an exemplary IMU 108 positioned generally at the vehicle's center of gravity and remote from the GNSS receiver electronics 106. In the system shown, the measurements from both the GNSS sensors and the inertial sensors are available for processing. The GNSS measurements can be used to calibrate the inertial instruments over time by updating estimates of inertial sensor parameters at the relatively slow GNSS update rate. However, in situations where the tracking of the GNSS signals is compromised by low SNR, extreme antenna acceleration, destructive multipath or similar interference, latency between the IMU processor and the GNSS processor will generally preclude calibrating in the reverse direction.
Alternatively, the IMU may be combined with the GNSS receiver electronics in a single enclosure. Such a configuration is shown in FIG. 1B. As shown, a processing unit 105 includes both GNSS receiver electronics 106a and IMU 108a. This topology offers several advantages over the topology of FIG. 1A. These include elimination of the communications harness between the IMU and GNSS receiver unit; reduction in communication latency and complexity; and synchronous sampling of GNSS and inertial measurements, allowing: high bandwidth inertial measurement flow into the GNSS tracking channels; low bandwidth processed GNSS measurements update inertial measurement parameters; and high bandwidth GNSS phase data are available for update of inertial instrument parameters.
However, such a topology also suffers from disadvantages related to the fact that the point at which the inertial sensors reside is physically remote from the phase centers of the antennas. The GNSS receiver 106 senses position (phase information) at the phase centers of its antennas. The IMU samples and integrates its internal sensors at the physical location of the IMU. Because the antennas and the IMU enclosure are physically separated, a projection algorithm must be applied before the measurements can be compared for purpose of complementary filtering. The projection algorithm requires the vectors between the IMU and the antenna phase centers be accurately known. This requirement can be problematic because it mandates an installation calibration procedure that may be complex; the vectors may change over time; and the vectors may change during operation, e.g., due to the structural flexibility of the vehicle, or elements of the vehicle.